A typical sewer system includes a main pipe and a series of lateral pipes communicating between buildings and the main pipe. Main pipes require periodic inspection and maintenance as the pipe ages or is subject to external destructive forces (e.g., frost, thermal cycles, earthquakes, and the ground shifting or sinking around the pipes). One major problem that may arise from a deteriorated main pipe is the flow of groundwater into the main pipe through cracks or improperly sealed joints. If large amounts of groundwater flow into a main pipe, the sewerage authority must process the groundwater along with the sewage and waste water for which the sewer system was intended. Such unnecessary processing of groundwater can be very expensive to the sewerage authority and, ultimately, the taxpayers.
It is therefore common practice to periodically inspect main sewer pipes for cracks or flaws in the grout that seals the joints of the main pipe sections and the junctions between the main and lateral pipes. When a crack or leaking joint is discovered, the usual practice is to seal the crack or joint with grout. If the main pipe has multiple cracks and is leaking substantial amounts of groundwater, it is common to install a rigid liner to improve the containment capability of one or more sections of the main pipe.
Such liners are intended to extend the useful life of the main pipe, but the liners also cover the lateral openings during the lining process. It is necessary to cut through the liner to reestablish communication between the main and lateral pipes. If the cutting is not executed accurately, the cutting device may cut through the liner and the main pipe walls and lower the containment capability of the main pipe. If the cutting device cuts too many erroneous holes in the liner and main pipe walls, the purpose of the liner is frustrated and the main pipe may have to be dug up and replaced.
It is known to map out the positions of lateral pipe openings in the main pipe with conventional measurement techniques. After the main pipe has been mapped and lined, a robotic router or cutting device is introduced into the main pipe. The cutting device usually includes a conventional visual camera that permits an above-ground operator to see the inside of the main pipe. The operator remotely navigates the cutting device to the lateral openings by using the map that was created prior to lining the main pipe, and also using the visual camera to look for dimples in the liner that in some cases indicate the presence of a lateral opening. At each expected lateral opening location, the operator causes the cutting device to cut a hole through the liner.
The lateral openings are typically only about 3 to 4 inches in diameter. It is often very difficult in the first place to accurately map the location of lateral pipe openings in the main pipe because both the axial and radial coordinates have to be taken into account. The visual camera is sometimes helpful to confirm to the operator the location of a lateral opening due to the presence of a dimple in the liner, but it is also not uncommon for the liner to have dimples that do not correspond to lateral openings or are not centered on the lateral opening.
An operator using a conventional system is therefore often left guessing at the exact location of a lateral opening and hoping that the cutting device cuts through the liner at a location that perfectly aligns with the lateral opening. Missing the lateral opening by even a small margin can compromise the main pipe's containment capability as the cutter cuts a hole in liner and main pipe wall.